Abstract: The results of an investigation and research into the cyanobacteria,which caused the water bloom in the Dianchi Lake from September 2001 to July 2002 is reported.The results showed that the cyanobecteria that caused the water bloom were Microcystis Kutz.and Aphanizomenon Morr.,and that the former was the dominant genus in the lake.The total concentration of the cyanobacteria causing water bloom was 3445000-1080800000 cell/L(cells per liter),and the annual concentration during this period was (130866603).These concentration of cyanobecteria which caused the water bloom accounted for 66.4%-(90.7%) of the total number of phytoplankton observed in the samples.The total concentration of genus Microsystis Kutz was 3445000-1030800000 cell/L,with annual average concentration of 123174582 cell/L,((35.7%-88.0%) of total concentration of phytoplankton).The density of Aphanizomenon was 0-5950000 cell/L,and the annual average concentration was 5692021 cell/L,(0.46%-30.75% of the total number of phytoplankton).The seasonal variation cyanobacteria causing the water bloom was as same as Microsystis Kutz.,that is the highest concentration was in July and the lowest in January.The presence of Microsystis Kutz.near the bank of the lake was higher than that in the center part,and also higher in the north than in the south.The density of Aphanizomenon Morr.was the highest in March and the lowest in January.Also the density of Aphanizomenon Morr.in the north part was higher than that in the south part of the lake,which was in turn higher than the center part of the lake.The water bloom of Microsystis Kutz.was observed during the whole year,but the water bloom of Aphanizomenon Morr.was only observed in March.